This invention relates to the field of quadrature modulated color television picture signal encoding, and more particularly to the field of the prefiltering of encoded video signals to minimize cross-color and cross-luminance or dot crawl artifacts when the encoded video signals are subsequently decoded.
FIG. 1 is a graph of the frequency spectrum occupied by a typical quadrature modulated color television picture signal in accordance with the NTSC format. As is shown, luminance and chrominance information share the same spectrum.
A typical NTSC encoder is shown in FIG. 2 in block diagram form. The R (red), G (green) and B (blue) signal inputs come from a camera and are each applied to three matrices. The outputs of these matrices are luminance, Y, and two chrominance components, I and Q. The luminance bandwidth is typically limited to 4.2 MHz, the I component bandwidth is typically limited to 1.3 MHz, and the Q component bandwidth is typically limited to 0.6 MHz. The I and Q components are then impressed as carrier suppressed amplitude modulation components in phase quadrature upon a subcarrier at about 3.58 MHz. This subcarrier frequency is selected to result in a 180.degree. phase shift from scanning line to adjacent scanning line, and from frame to frame, within the color television picture signal.
The quadrature modulated subcarrier is then added to the luminance carrier, and the resultant composite video signal is low pass filtered to 4.2 MHz. Addition of composite synchronization pulses, proper blanking, pedestal adjustments etc., results in a signal in accordance with the NTSC format.
Referring to FIG. 3, a detailed view of the frequency spectrum of an NTSC encoded color picture signal in the vicinity of the subcarrier shows the principle of "interleaving". Spectral rays of a typical television scene are grouped around integer multiples of the horizontal scanning frequency fh for the luminance information, while chrominance components are grouped around (2n+1)/2 fh (where n is an integer). For vertical components of a picture, this grouping is a very accurate representation of their spectral appearance, and separation of the chrominance and luminance components can be accomplished at the receiver using a comb filter without major difficulty.
However, the frequency interleaving is perfect only in the case of vertical transitions in the picture. Diagonal and horizontal transitions in the picture manifest an overlap of chrominance and luminance spectra, and the separation of the luminance and chrominance components in the receiver becomes much more difficult.
As a result of this imperfect separation of the luminance and chrominance components, certain luminance components are interpreted by the decoder in the receiver and decoded as color, resulting in "cross-color" patterns that are typically perceived as a moving rainbow accompanying diagonal luminance transitions, or as other inappropriate color activity in the proximity of luminance details.
Conversely, certain chrominance components are also interpreted by the receiving decoder and decoded as luminance. One or two lines of dots at the 3.58 MHz color subcarrier frequency are frequently noticeable in the luminance path for horizontal chrominance transitions derived from comb filter decoders. One possible solution is to place a 3.58 MHz trap in the luminance path, but doing so creates a poor frequency response, or a vertical dot pattern with vertical chrominance transitions, or both.
To prevent these problems at the receiver, it has become popular to process luminance and chrominance components by comb filter structures prior to their addition in an NTSC (or PAL) quadrature modulation encoder. As shown in FIG. 4, the frequencies that are in the vicinity of the chrominance subcarrier frequency are combed in the luminance path, while the entire chrominance spectrum is combed in the chrominance path. The circuit shown in FIG. 4 is fairly effective in reducing cross-color and cross-luminance artifacts. The spectral results achieved by this comb filter process are shown in FIG. 5A.
FIG. 5B shows the spectrum produced by an improved digital comb filter having additional delay elements. As delay elements are added, the steeper slopes on the "teeth" of the comb filter outputs result in decreased spectral overlap, as shown in FIGS. 5B and 5C.
In a two-dimensional digital comb filter with two increments of delay, luminance frequencies within the band of the chrominance subcarrier are averaged over three lines, and two adjacent pixels. Such a filter produces a negligible output for horizontal and vertical lines and edges. However, if a transition in luminance, such as an edge or line, is at 45.degree., it creates a ringing effect that disturbs the values in adjacent pixels along the transition.
U.S. Pat. No. 4,731,660 (Faroudja et al) hereby incorporated by reference, discloses one approach to reducing cross-color and cross-luminance patterns in the encoding of quadrature modulated color television picture signals by a means that is somewhat selective and therefore preserves more picture detail.
In this approach, referring to FIG. 6, the luminance component is applied to a comb filter 58 to produce a comb filtered luminance component, which is bandpass limited by a bandpass filter 56. The luminance component is also applied to a delay matching circuit 62. The output of the bandpass filter 56 is applied to both threshold circuit 78 and a rectifier/integrator 92. The output of rectifier/integrator 92 is a signal 94 that is representative of the level of cross-color activity over a recent interval, e.g. one microsecond.
This cross-color activity signal 94 is applied to a combiner circuit 95, along with another signal 80 that is representative of the chrominance activity. The output of the combiner circuit 95 is a control signal for the threshold circuit 78. The threshold circuit output is subtracted by a subtractor 60 from the delayed luminance component arriving from the delay matching circuit 62.
Whenever the levels of cross-color activity and chrominance activity are high, indicating that correction is required, the threshold control signal 98 adjusts the threshold circuit 78 and increasing
quantities of comb filter 58 output are subtracted from the delayed luminance signal. Conversely, whenever the levels of cross-color activity and chrominance signal are low, indicating that minimal correction is needed, the threshold control signal 98 adjusts the output of the limiter to cause less energy to be subtracted from the delayed luminance signal. Thus, correction is only selectively applied at those times that the cross-color and chrominance level signals indicate that there is a problem requiring it, thereby reducing the amount of detail that is lost in the compensation process.
In co-pending patent application Ser. No. 07/294,235 (now U.S. Pat. No. 4,951,129), hereby incorporated by reference, another approach to reducing cross-color and cross-luminance patterns in the encoding of quadrature modulated color television picture signals is proposed. This application describes an improvement on the simple comb filter generally employed for detecting and compensating for cross-color and cross-luminance patterns.
In this approach, both the luminance and chrominance components are filtered using a two-dimensional (horizontal and vertical) filter, a vertical only filter, and a horizontal only filter as shown in FIGS. 7A-7C, respectively. A correction value is generated based on the outputs of each of these filters. For each pixel, the correction values are compared in a two-stage decision making process to decide which one to apply as the actual correction factor.
Referring to FIG. 7A, the first of these filters processes five adjacent pixels horizontally along three consecutive lines of video signal. The current pixel is in the center, and indicated by a heavier border. The arrows above this filter show how every other sample of luminance information (Y) for a given line corresponds to the same chrominance component, in this case Q. The numbers in each filter box indicate the weights that are applied to each pixel. All of the weighted values are added together and divided by either 32 or 16, depending on other circuit values and how strong a correction factor is desired.
The output of the two-dimensional filter, Y.sub.cc, will be near zero in the absence of diagonal lines or edges. When a diagonal line or edge passes through the filter, however, Y.sub.cc assumes a value significantly greater than zero. Since diagonal lines and edges produce the greatest cross-color artifacts, Y.sub.cc is a good measure of their presence. As will be seen below in the discussion of the decision making process that uses the output of all three filters, the value Y.sub.cc is used to limit the filtering that is actually applied to the luminance signal.
The second, vertical filter is shown in FIG. 7B. It examines the relationship between the current pixel, shown in the darker central box, and the same pixel in adjacent lines vertically. The output of the vertical filter is Y.sub.cv.
The third, horizontal filter is shown in FIG. 7C. It examines the relationship between the current pixel, shown in the darker central box, and the adjacent horizontal pixels in the same line. Again, only luminance pixels related to the same chrominance component are used, as was shown in FIG. 7A. The output of the horizontal filter is Y.sub.ch.
The decision process is illustrated in FIGS. 8 and 9. A vertical edge or line results in a large value of Y.sub.ch, but small values of both Y.sub.cv and Y.sub.cc. Conversely, a horizontal edge or line results in a large value of Y.sub.cv, but small values of both Y.sub.ch and Y.sub.cc. Diagonal edges produce large values for both Y.sub.cv and Y.sub.ch, but a smaller value for Y.sub.cc. Diagonal lines produce relatively large values for all three filter outputs, but Y.sub.cc is generally the smallest. The decision process uses the smallest of these correction values or zero, thus limiting the prefiltering of the picture to only those pixels that require it and to the minimum magnitude necessary.
The first step in the decision making process is to determine an intermediate value Y.sub.cx based on the values of Y.sub.cc, Y.sub.cv, and Y.sub.ch. As shown in FIG. 8, if Y.sub.cc is a negative number, the more positive value of Y.sub.cv or Y.sub.ch is assigned to be the intermediate correction value, Y.sub.cx. Y.sub.cx may be either positive or negative. If Y.sub.cc is non-negative, the more negative value of Y.sub.cv or Y.sub.ch is assigned to be Y.sub.cx.
Referring now to FIG. 9, the second step in this two stage decision process is to find a final correction value Y.sub.cf that is limited to be between zero and Y.sub.cc. If Y.sub.cx is already between Y.sub.cc and zero, then Y.sub.cx is used as the final correction value Y.sub.cf. Otherwise, Y.sub.cx is clipped to Y.sub.cc or zero to produce Y.sub.cf. The final correction value Y.sub.cf is added to the current pixel when it arrives in the delayed luminance path to accomplish the prefiltering. This decision process attempts to keep the softening of the luminance signal to a minimum, while effectively removing the information that produces cross-color error.
Both of the approaches described above, the variable threshold of the U.S. Pat. No. 4,731,660 and the adaptive filter of the co-pending patent application Ser. No. 07/294,235 now U.S. Pat. No. 4,951,129, while improvements over the prior art as represented by FIG. 4, still leave something to be desired, in that they still tend to overreact to the presence of false indications of cross-color and dot-crawl problems, excessively dampening image detail. Even more discrimination in eliminating cross-color and cross-luminance artifacts is desired.